In a magnetic disk drive for a disk such as a hard disk, servo data is recorded along tracks formed on a disk medium, and the servo data is read by a head flying on the disk medium. Hence, the magnetic disk drive includes therein a feedback control system as a head position control system that controls the position of the head to follow tracks in correspondence to the servo data.
In many cases, non-linearity elements are included in a head position control system, such as described above, so identification of the non-linearity of the system is required. For example, suppose that an attempt is made to inhibit a write operation in correspondence to the position of the head. In this case, when an error signal (PES: position error signal) from which the head position is obtained is being under the influence of non-linearity, a correct position cannot be acquired from the head, and hence it is difficult to correctly inhibit the write operation. As such, the non-linearity in the position control system has to be identified to create a function value that compensates for the non-linearity.
In a normal case, in a system without feedback, a reference signal is input into the system, and a signal output from the system is compared with the reference signal, thereby to make it possible to identify the non-linearity in the system.
Nevertheless, however, in the head position control system, when the reference signal is input, a signal influenced by the non-linearity is fed back, so that non-linearity cannot be appropriately identified in accordance with the output signal.